A wide variety of memory devices can be used to maintain and store data and instructions for various computers and similar systems, including non-volatile memory devices that do not require power to retain information (e.g., EPROM, EEPROM, and Flash memory).
Memory devices contain memory cells, which store information in the form of binary bit(s) (e.g., logic state “0”, “1”, “00”, “01”, “10”, or “11”). Conventional technology senses the logic state of memory cells utilizing fixed and/or dynamic tracking references, comparing a sensed current/voltage with a fixed and/or dynamic tracking reference current/voltage. For example, if the sensed current drawn by a memory cell exceeds a reference current, the memory cell's logic state is evaluated as a logic 1; otherwise, the memory cell's logic state is evaluated as a logic 0.
One concern with conventional memory sensing technology is that fixed references may not correctly evaluate the logic state of a memory cell when one or more characteristics of the memory cell (e.g., threshold voltage, drain current) change and/or degrade over time. For example, if at the beginning of life of a memory cell, the current sensed during a read operation of the memory cell exceeds a fixed reference current, the logic state of the memory cell is evaluated as a logic 1. However, if characteristics of the memory cell change after a period of time, causing current sensed during a read operation of the memory cell to be below the fixed reference current, the logic state of the memory cell is evaluated as a logic 0—data stored in the memory cell cannot reliably be determined. Therefore, the usable life of a memory cell terminates when a fixed reference cannot account for changes in the memory cell's characteristics.
Another concern with conventional memory sensing technology is that dynamic tracking references may not correctly evaluate the logic state of a memory cell when one or more characteristics of the memory cell shift over time—a memory cell's characteristics can change inconsistently compared to characteristics of a dynamic tracking reference. Therefore, when a fixed reference cannot account for changes in the memory cell's characteristics, data stored in the memory cell cannot reliably be determined. Therefore, the usable life of a memory cell terminates when a fixed reference cannot account for changes in the memory cell's characteristics.
It is therefore desirable to have systems and methods that can extend the usable lifetime of memory cells by avoiding limitations of conventional sensing techniques.